public virtual new GeometryBag ToGeometry()

MapPoint point = MapPointBuilderEx.CreateMapPoint(1, 2, SpatialReferences.WebMercator);

List<Coordinate2D> coords2D = new List<Coordinate2D>()
{
  new Coordinate2D(0, 0),
  new Coordinate2D(0, 1),
  new Coordinate2D(1, 1),
  new Coordinate2D(1, 0)
};

Multipoint multipoint = MultipointBuilderEx.CreateMultipoint(coords2D, SpatialReferences.WGS84);
Polyline polyline = PolylineBuilderEx.CreatePolyline(coords2D, SpatialReferences.WebMercator);

GeometryBagBuilderEx builder = new GeometryBagBuilderEx(SpatialReferences.WGS84);

GeometryBag emptyBag = builder.ToGeometry();
// emptyBag.IsEmpty = true

builder.AddGeometry(point);
// builder.GeometryCount = 1

GeometryBag geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 1
// geometryBag.PointCount = 1
// geometryBag.IsEmpty = false

IReadOnlyList<Geometry> geometries = geometryBag.Geometries;
// geometries.Count = 1
// geometries[0] is MapPoint with a sr of WGS84

bool isEqual = geometryBag.IsEqual(emptyBag);   // isEqual = false

builder.InsertGeometry(0, multipoint);
geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 2

geometries = geometryBag.Geometries;
// geometries.Count = 2
// geometries[0] is Multipoint
// geometries[1] is MapPoint

builder.AddGeometry(polyline);
builder.RemoveGeometry(1);
geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 2

geometries = geometryBag.Geometries;
// geometries.Count = 2
// geometries[0] is Multipoint
// geometries[1] is Polyline          

// Use a builder convenience method or use a builder constructor.

MapPoint point = MapPointBuilderEx.CreateMapPoint(10, 20);
List<Coordinate2D> coords = new List<Coordinate2D>() { new Coordinate2D(50, 60), new Coordinate2D(-120, -70), new Coordinate2D(40, 60) };
Multipoint multipoint = MultipointBuilderEx.CreateMultipoint(coords, SpatialReferences.WebMercator);
Polyline polyline = PolylineBuilderEx.CreatePolyline(coords);

string json = "{\"rings\":[[[0,0],[0,1],[1,1],[1,0],[0,0]],[[3,0],[3,1],[4,1],[4,0],[3,0]]]}";
Polygon polygon = PolygonBuilderEx.FromJson(json);

var geometries = new List<Geometry>() { point, multipoint, polyline, polygon };

// Builder convenience methods don't need to run on the MCT.
//At 2.x - GeometryBag bag = GeometryBagBuilder.CreateGeometryBag(geometries, SpatialReferences.WGS84);
var bag = GeometryBagBuilderEx.CreateGeometryBag(geometries, SpatialReferences.WGS84);

//At 2.x - using (var builder = new GeometryBagBuilder(geometries, SpatialReferences.WGS84)) 
var builder = new GeometryBagBuilderEx(geometries, SpatialReferences.WGS84);
// do something with the builder
bag = builder.ToGeometry();

MapPoint point = MapPointBuilderEx.CreateMapPoint(10, 20);
List<Coordinate2D> coords = new List<Coordinate2D>() { new Coordinate2D(50, 60), new Coordinate2D(-120, -70), new Coordinate2D(40, 60) };
Multipoint multipoint = MultipointBuilderEx.CreateMultipoint(coords, SpatialReferences.WebMercator);
Polyline polyline = PolylineBuilderEx.CreatePolyline(coords);

string json = "{\"rings\":[[[0,0],[0,1],[1,1],[1,0],[0,0]],[[3,0],[3,1],[4,1],[4,0],[3,0]]]}";
Polygon polygon = PolygonBuilderEx.FromJson(json);

var geometries = new List<Geometry>() { point, multipoint, polyline, polygon };

//At 2.x - using (var builder = new GeometryBagBuilder(SpatialReferences.WGS84))
var builder = new GeometryBagBuilderEx(SpatialReferences.WGS84);
builder.AddGeometries(geometries);

GeometryBag geomBag = builder.ToGeometry();
// geomBag.PartCount == 4    (point, multipoint, polyline, polygon)

geometries = new List<Geometry>() { point, polyline };
builder.InsertGeometries(1, geometries);
// builder.GeometryCount == 6
geomBag = builder.ToGeometry();
// geomBag.PartCount == 6    (point, point, polyline, multipoint, polyline, polygon)